The present invention relates to an improved biological filter system and to a process and apparatus for the biological treatment of waste water.
It is known to decompose by bacteriological action organic materials which are dissolved in a waste water, by passing the waste water through a biological filter bed.
U.S. Pat. No. 3,928,190 discloses a biological filtering treatment system wherein a biological filter bed is formed of a granular material, such as baked clay, for example fire clay, having a grain size of from 0.5 to 4.0 mm, preferably approximately 2.0 mm, with the granular material having on the surfaces thereof hollow zones capable of supporting a bacterial film and retaining such bacterial film even when the filter bed is subjected to severe backwashing and smooth zones capable of supporting a bacterial film but permitting removal of such bacterial film during backwashing. This filter bed is used in an aerobiosis system for the biological purification of waste water, wherein the water entering the filter bed has dissolved therein oxygen in quantities as close to the saturation level as possible. This is done through preliminary aeration of the waste water, just prior to passage through the filter bed, by means of a diffusion device. By the use of this prior system, and by diffusing air into the waste water, it is possible to achieve dissolved oxygen concentrations of from approximately 7 to 8 mg/l. These values may be increased to approximately 25 to 30 mg/l by injecting pure oxygen or oxygen enriched air, rather than normal air, into the waste water.
It has been observed that when purifying waste water under the conditions set forth in U.S. Pat. No. 3,928,190, there is required only a relatively low amount of oxygen to remove the pollutants from the waste water, the degree of pollution in the waste water conventionally being expressed in terms of biochemical oxygen demand (BOD.sub.5), which is the quantity of oxygen in milligrams per liter utilized in the biological oxidation of the organic matter contained in the waste water within a period of five days at 20.degree. C. Thus, it is readily determined that the elimination of pollution in the amount of 1 mg of BOD.sub.5 requires only that approximately 0.5 to 0.7 mg oxygen be dissolved in the waste water prior to its passage through the biological filter. Thus, by preliminary aeration of the waste water with air before passage of the waste water through the filter of U.S. Pat. No. 3,928,190, it is possible to eliminate approximately 10 to 16 mg BOD.sub.5 per liter of filtered water. The amount of eliminated pollution can be raised to approximately 40 to 50 mg/l when the water waste is preliminarily aerated with pure oxygen.
However, when urban or industrial waste water to be treated is subjected to a preliminary purification involving the addition of flocculating reagents and then the separation of the flocculated products, or when it is desired to directly purify an industrial waste water which is only slightly polluted as compared with normal industrial waste water, it might be expected that the amount of pollution dissolved in the waste water at the inlet of the filter might easily attain or even exceed 100 mg/l of BOD.sub.5. If this higher amount of initial pollution is to be reduced to a level of approximately 30 mg/l by use of the biological filter system of U.S. Pat. No. 3,928,190, then the amount of oxygen which can be dissolved into the waste water is insufficient to achieve such degree of purification. In U.S. Pat. No. 3,928,190, this deficiency in dissolved oxygen content is at least partially relieved by recycling a portion of the filtered water upstream of the aeration device, to thereby increase the amount of available oxygen. However, when this recycling operation is necessary, then the overall filtering production rate is limited to a value of approximately 8 to 13 m.sup.3 /m.sup.2 /h. Therefore, to avoid a loss in production efficiency, it is oftentimes necessary to increase the dimensions of the filter. It will accordingly be apparent that the amount of pollution which can be eliminated per unit volume of the filter bed is limited. For example, for a filter bed height of between 1.5 and 2.0 meters, the amount of pollution which can be eliminated per unit of volume of the filter bed is approximately 2 kg of BOD.sub.5 per cubic meter of filter bed per day when the preliminary aeration is achieved with normal air, and approximately 4 to 5 kg of BOD.sub.5 per cubic meter of filter bed per day when the preliminary aeration is achieved by pure oxygen or oxygen enriched air.
A further biological filter system for the purification of waste water is shown in U.S. Pat. No. 4,053,396, wherein there is employed an activated carbon filter bed, wherein the waste water has dissolved therein oxygen in an amount in the range of from 0.09 to about 0.15 pound of oxygen consumed per pound of total COD (chemical oxygen demand) of contaminants removed from the waste water. The oxygen is added to the waste water in the head space of each filter bed, but the last filter bed of a plurality of serially connected filter beds. However, in the system of U.S. Pat. No. 4,053,396, the control of the amount of oxygen dissolved in the waste water is achieved by plural flow rates of oxygen added at different stages of the system. Such prior art system is however complicated to control and regulate, particularly when the level of pollution in the waste water supplied to the system continually varies, as is conventional.